The IOR rule explained:

Sailboats racing under a ``handicap system'' have a function applied to their elapsed time, producing a ``corrected time,'' and the boats place in corrected time order. This function, which differs among systems, attempts to fairly represent speed differences among boats.

There are two major handicapping philosophies: ``measurement'' rules which handicap based upon measurements, and ``rating'' rules which handicap based upon observed performance.

The International Offshore Rule (IOR) is a measurement rule for racing boats. The IOR evolved from the Cruising Club of America (CCA) rule for racer/cruisers.

The IOR concentrates on hull shape with length, beam, free board and girth measurements, foretriangle, mast and boom measurements, and stability with an inclination test.

The IOR also identifies features which are dangerous or it can't fairly rate, and penalizes or prohibits them.

The measurements and penalties are used to compute the handicap number which is an ``IOR length'' in feet. A typical IOR 40 footer (a ``one tonner'') has rating of 30.55 feet.

In a handicapped race, the IOR length is used to compute a ``time allowance,'' in seconds per nautical mile (s/M) which is multiplied by the distance of the race, and subtracted from the boat's actual time, to compute the boat's corrected time. Longer IOR length gives a smaller time allowance.

The IOR is (also) used to define "level classes,'' where no time correction is used. Every boat in a class has an IOR number less than some number. The Ton Classes, (Mini Ton, 1/4 Ton, 1/2 Ton, 3/4 Ton, 1 Ton, and Two Ton), as well as 50-footer, ULDB 70 and Maxi classes are examples.

To account for improvements in design and materials, boats are given an ``old age allowance'' which decreases their IOR length as time passes. In spite of the old age allowance, about 3/4 s/M/year on 40 footer, boats over several years old are usually not competitive, which is why IOR handicap racing is dead.

Peculiarities of IOR designs result from features which increase actual performance more than they increase IOR length, or other odd rules; IOR hulls bulge at girth measurement points; a reverse transom moves a girth measurement point to a thicker part of the hull; waterline length is measured while floating upright, so large overhangs are used to increase waterline sailing at speed; the stability factor ignores crew, so IOR designers assume lots of live ballast; after the 1979 Fastnet race excessive tenderness was penalized; full length battens were prohibited to prevent main sail roach area, but short battens became strong enough that the IOR had to start measuring and penalizing extra main sail girth; main sail area adds less IOR length than jib area, so new IOR boats are fractionally rigged; The IOR encourages high free board, and high booms and prohibits keels wider at the bottom than at the top (bulbs).